The subject invention relates generally to the compositions of wallboard, also known as “drywall”, and methods of the preparation of wallboard. More particularly, the subject invention is generally directed towards methods of preparation of a wallboard composition that has improved strength characteristics when the amount of water in the wallboard composition is reduced. Additionally, the subject invention relates to the manufacturing process used to produce wallboard compositions with such improved strength characteristics.
Conventional gypsum wallboard has been used for over fifty years in the construction of residential and commercial building interior walls and ceilings. Typically, wallboard consists essentially of a gypsum core sandwiched between and bonded to two sheets of paper and is used as a cost-effective replacement of conventional plaster walls. Walls and ceilings made with gypsum wallboard panels are conventionally constructed by securing, with nails or screws, the wallboard panel to structural members, such as wood or metal studs. To be commercially profitable, wallboard is typically manufactured by continuous high speed processes. Typically, natural gypsum (calcium sulfate dihydrate) predominately makes up the wallboard. Manufactures mine and transport gypsum to a board mill in order to dry it, crush/grind it and calcine it to yield stucco. The reaction for the calcinations process is characterized by the following equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2OThis equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is conducted in a calciner, of which there are several types known in the art.
Calcined gypsum, stucco, has the valuable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction reverses the above-described stucco chemical reaction performed during the calcination step. The reaction proceeds according to the following equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heatIn this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction generally depends upon the type of calciner employed and the type of gypsum rock that is used. The reaction time can be controlled to a certain extent by the use of additives such as accelerators and retarders.
While conventional gypsum wallboard has many advantages, it has long been desired to reduce the cost of manufacturing such wallboard. One method of reducing the cost of wallboard has been to reduce the amount of water used in the manufacturing of the wallboard. Reduction in water allows for less drying energy which in turn saves the energy costs associated with drying wallboard (i.e., the fuel cost associated with operating a kiln to dry the wallboard). The problem associated with reducing water in the manufacturing process has been keeping the slurry, as explained below in more detail, fluid enough to allow it to be continually deposited on the cover paper. Reducing water negatively impacts the manufacturing process by reducing the slurry fluidity, increasing board weight, adversely affecting the paper-to-core bond, and decreasing the compressive strength of the board. The decrease in wallboard strength and bond quality from reduced water levels has prevented manufacturers from further cutting manufacturing costs by reducing the amount of water used in the slurry composition.
Thus, it would be advantageous to produce a gypsum wallboard composition with improved strength characteristics that allowed for the reduction of the amount of water used in producing the wallboard. Moreover, it would be advantageous to produce a high-strength gypsum wallboard composition that can be combined with known methods of producing lightweight gypsum wallboard in order to impart on such lightweight wallboards strengths similar to, or greater than, heavier wallboard. Such wallboard also should be able to be manufactured through the use of conventional high-speed manufacturing processes without suffering from any negative side effects.